U.S. patent application number 13/806130 was filed with the patent office on 2013-04-25 for energy storage system.
This patent application is currently assigned to NATIONAL UNIVERSITY OF SINGAPORE. The applicant listed for this patent is Tanmoy Battacharya, Ashwin M. Khambadkone, Haihua Zhou. Invention is credited to Tanmoy Battacharya, Ashwin M. Khambadkone, Haihua Zhou.
Application Number | 20130099581 13/806130 |
Document ID | / |
Family ID | 45371688 |
Filed Date | 2013-04-25 |
United States Patent
Application |
20130099581 |
Kind Code |
A1 |
Zhou; Haihua ; et
al. |
April 25, 2013 |
Energy Storage System
Abstract
An energy storage system comprises a plurality of storage
mediums having substantially different energy and power density
that are each connected to a DC bus via a respective bidirectional
isolated DC-DC converter; and a controller configured to
independently determine a current demand for each storage medium
based on a control mode.
Inventors: |
Zhou; Haihua; (El Segundo,
CA) ; Battacharya; Tanmoy; (Kharagpur, IN) ;
Khambadkone; Ashwin M.; (Singapore, SG) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Zhou; Haihua
Battacharya; Tanmoy
Khambadkone; Ashwin M. |
El Segundo
Kharagpur
Singapore |
CA |
US
IN
SG |
|
|
Assignee: |
; NATIONAL UNIVERSITY OF
SINGAPORE
Singapore
SG
|
Family ID: |
45371688 |
Appl. No.: |
13/806130 |
Filed: |
June 21, 2011 |
PCT Filed: |
June 21, 2011 |
PCT NO: |
PCT/SG2011/000219 |
371 Date: |
December 20, 2012 |
Current U.S.
Class: |
307/82 |
Current CPC
Class: |
Y02B 10/30 20130101;
H02J 7/34 20130101; H02J 1/10 20130101; H02J 1/001 20200101; H02J
7/345 20130101; H02J 1/12 20130101 |
Class at
Publication: |
307/82 |
International
Class: |
H02J 1/10 20060101
H02J001/10 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 21, 2010 |
SG |
201004447-7 |
Claims
1. An energy storage system comprising: a plurality of storage
mediums having substantially different energy and power density
each connected to a DC bus via a respective bidirectional isolated
DC-DC converter; and a controller configured to independently
determine a current demand for each storage medium based on a
control mode.
2. The system in claim 1 wherein a high frequency current demand is
determined for at least one of the storage medium with a relatively
fast response and a low frequency current demand is determined for
at least one of the storage medium with a relatively slow
response.
3. The system in claim 2 wherein the capacity of the fast response
storage medium is configured to support a desired power
density.
4. The system in claim 3 wherein the fast response storage medium
is an ultracapacitor.
5. The system in claim 2 wherein the capacity of the slow response
storage medium is configured to support a desired energy
density.
6. The system in claim 5 wherein the slow response storage medium
is one or more batteries.
7. The system in claim 1 wherein the storage mediums are selected
from the group consisting of batteries, ultracapacitors, fuel
cells, electrolytics, fly wheels and any combination.
8. The system in claim 1 wherein the bidirectional isolated DC-DC
converters are dual active bridges (DAB) configured to be connected
in a plurality of predetermined configurations for different DC bus
voltages.
9. The system in claim 8 wherein a plurality of DABs connected to a
plurality of batteries are connected in a modified input parallel
output series configuration, a plurality of DABs connected to an
ultracapacitor are connected in an input parallel output series
configuration and the two pluralities of DBAs are intermittently
bridged.
10. The system in claim 9 wherein each DBA includes a high
frequency transformer.
11. The system in claim 9 wherein the DABs configured to interleave
a plurality of switching instants to reduce an output ripple.
12. The system in claim 2 wherein the low frequency current demand
is determined using a low pass filter with a cut-off frequency
determined based on a relative capacity of the fast response
storage medium compared to the slow response storage medium.
13. The system in claim 1 wherein the control modes are selected
from the group consisting of dynamic allocation of current demand
between the storage mediums, state of charge balancing between the
storage mediums, slow response storage medium removal, fast
response storage medium charging and any combination.
14. The system in claim 2 wherein the low frequency current demand
for the slow response storage medium is controlled using a current
feedback loop.
15. The system in claim 2 wherein the high frequency current demand
for the fast response storage medium is fed forward to the output
of a voltage feedback loop.
16. An energy apparatus comprising; an energy generation system,
and an energy storage system according to claim 1, adjacent to the
energy generation system and configured to absorb any excess power
and supply any shortfall power from the energy generation
system.
17. The system in claim 16, wherein the energy apparatus is
selected from the group consisting of a micro-grid, an isolated or
standalone electrical system and an electric vehicle.
18. The system in claim 16, wherein the energy generation system is
selected from the group consisting of a PV solar array, a wind
turbine, micro hydro turbine, combined cycle turbine, and any
combination.
19. A method of distributing current between a plurality of energy
storage mediums connected between a generator and a load
comprising: determining an overall current demand based on an
instantaneous generation power from the generator and an
instantaneous load power from the load; controlling a current a
slow response energy storage medium based on a low frequency
component of the overall current demand; and controlling a current
of a fast response energy storage medium based on a high frequency
component of the overall current demand.
Description
FIELD
[0001] The present invention relates to an energy storage
system.
BACKGROUND
[0002] Today's electricity grid relies on a number of different
sources of power generation. Traditional sources include coal fired
turbines, gas fired turbines, oil fired turbines, hydro dams,
nuclear reactors and the like. However as shown in FIG. 1, if
energy demand increases by 5% each year fossil fuel reserves 100
will run out by approximately 2050. As a result alternative energy
sources and more environmentally friendly generation techniques are
coming into favour, including wind farms, solar farms, tidal
generators, etc. However both traditional and green generation
suffer from the same problem, that they need to be located remotely
a long way from where the demand is. Moreover, they suffer from
intermittency.
[0003] For traditional generation the remote location is required
either due to pollution concerns or the large amount of space
required. For green generation often the natural resource is in the
remote location, or it is due to the large amount of space
required. In either case the distance the electricity needs to be
transmitted causes difficulty in terms of the cost of the
infrastructure and the electrical losses that may result. Also more
reliance on fewer large scale generation sources may lead to lower
reliability for the system and less stability.
[0004] One option is smaller generation plants closer to the demand
centres. However it is difficult to situate even small generation
plants close to population centres without raising noise, pollution
or risk concerns with the inhabitants.
[0005] At the other end of the scale it is has also been proposed
to integrate green energy generation into buildings and in some
cases to design the building to have zero energy consumption
overall. FIG. 2 shows a comparison 200 between the typical solar
Photovoltaic (PV) generation profile 202 over 24 hours and the
building load profile 204. This shows a temporal mismatch where
sometimes the grid is required to supply 206 or absorb 208 energy
at certain times of the day.
[0006] When generation does not match demand, the frequency of the
grid tends to vary. This needs regulation. Regulating reserves are
put in use to overcome the problems. Such reserves are fast
responding power plants on stand by. However, the response time
required when renewable are used in generating mix is very small.
Due to moving cloud, the solar PV output can drop very quickly and
change to a new value causing a fast large increase in grid load.
Similarly during peak sunshine hours there will be an large excess
of supply from the PV array (i.e. the building appears as a
generator not a load). If either were to happen in an aggregated
form, a fast acting regulating reserve would be needed to keep the
grid frequency within mandatory limits. Thus if building integrated
PV arrays become highly popular this may cause challenges for the
grid.
[0007] Another complication is that the metering and billing system
might have to be able to cope with two way power flow. Also power
systems need to be rated off the peak power requirements, even with
building integrated generation, the remote generation and grid
capacity might need to be rated for if there was no building
integrated generation. As an example if the whole city was covered
by cloud (and the building integrated generation was all solar) the
shortfall would have to come from traditional generation. Thus the
system might need generation capacity (including that of the
building integrated generation) twice that of the peak demand, for
redundancy (this could be somewhat reduced by diversity into wind,
micro hydro, geothermal etc).
[0008] Thus it would be desirable for such green generation
systems, particularly those building integrated, to incorporate
some mechanism to avoid the demand/supply mismatches being
propagated into the grid.
[0009] Several attempts have been made to avoid the problems
mentioned. The simplest is a battery bank. As can be seen in FIG. 3
the Ragone plot 300 shows batteries 302 have high energy density
304 but suffer from the disadvantage that their power density 306
is not very high. High energy density 304 means that the steady
state capacity for energy storage is relative high. Low power
density 306 means that high frequency variations such as changes in
demand that occur over milli seconds cannot be supplied or absorbed
by battery bank 302. Thus with a battery bank 302 high frequency
variations will still be propagated to the grid.
[0010] Similarly other energy storage mediums suffer from problems
associated with either energy density 304 or power density 306.
Capacitors 308, fuel cells 310 and electrolytic capacitors 312 are
all shown in disparate locations in the plot 300 which do not
intersect.
[0011] Also since gaps 314 exist in the Ragone plot 300, some
scenarios of load profile and PV profile may not be able to be
accommodated with current technology.
[0012] Prior art attempts to solve this include US patent
publication numbers 2007/0062744 and 2010/0133025 which propose a
composite energy storage system which combines a battery bank and a
capacitor bank. However they suffer from the disadvantage that it
is a rigid configuration that may not be efficient or optimal in
certain circumstances such as the power and energy requirements of
a smart micro-grid.
[0013] This problem may be exaggerated in an island or an isolated
system not connected to the grid. In that case all load demands
must always be provided by the green generation system and/or an
energy storage system, both high frequency variation in the load
and/or generation and steady state.
SUMMARY
[0014] In general terms the invention proposes a composite of
different types of electrical energy storage mediums, where current
for each medium can be independently controlled depending of the
dynamic capacity of each medium, the DC voltage can be easily
configured for different AC load configurations and/or where energy
storage modules can be "hot swapped" in with a plug and play
capability. This may have one or more of the advantages of easy
reconfigurability of output, flexibility of storage mediums,
optimised size/weight, scalability, longer life, higher efficiency,
better power quality, plug and play capability modularity, active
battery charge balancing or distributing and/or lower cost.
[0015] In a first specific aspect the invention provides an energy
storage system comprising: [0016] a plurality of storage mediums
having substantially different energy and power density each
connected to a DC bus via a respective bidirectional isolated DC-DC
converter; and [0017] a controller configured to independently
determine a current demand for each storage medium based on a
control mode.
[0018] In a second specific aspect the invention provides an energy
apparatus comprising; [0019] an energy generation system, and
[0020] an energy storage system, adjacent to the energy generation
system and configured to absorb any excess power and supply any
shortfall power from the energy generation system.
[0021] In a third specific aspect the invention provides a method
of distributing current between a plurality of energy storage
mediums connected between a generator and a load comprising: [0022]
determining an overall current demand based on an instantaneous
generation power from the generator and an instantaneous load power
from the load; [0023] controlling a current a slow response energy
storage medium based on a low frequency component of the overall
current demand; and [0024] controlling a current of a fast response
energy storage medium based on a high frequency component of the
overall current demand.
[0025] Embodiments may be optionally be implemented according to
any one of claim 2 to 15, 17 or 18.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] FIG. 1 is a graph of predicted load growth compared to world
reserves of fossil fuels;
[0027] FIG. 2 is a graph of instantaneous power generation and
demand for a prior art building integrated PV system;
[0028] FIG. 3 is a Ragone plot of different storage mediums;
[0029] FIG. 4(a) is schematic diagram showing an energy system
according to an example embodiment;
[0030] FIG. 4(b) is a schematic showing the physical installation
of the components in FIG. 4(a);
[0031] FIG. 4(c) is a schematic showing the physical installation
of the components in FIG. 4(a);
[0032] FIG. 5 is a Ragone plot of the operating areas of the system
in FIG. 4;
[0033] FIG. 6 is a graph of a typical load profile indicating peak
power and overall energy requirements;
[0034] FIG. 7 is a graph comparing the weight of various
alternatives for energy storage including an embodiment;
[0035] FIG. 8 is a block diagram of the energy system in FIG.
4;
[0036] FIG. 9 is a circuit diagram of the DC-DC converter in FIG.
8;
[0037] FIGS. 10(a)-(h) are different connection configurations for
the converter in FIG. 9;
[0038] FIG. 11 shows FIG. 10(d) in more detail;
[0039] FIG. 12 is a table of the switching stresses in the
converter in FIG. 9;
[0040] FIGS. 13(a) and (b) are graphs of the input and output
ripple of the CESS in FIG. 11;
[0041] FIGS. 14(a)-(c) are logic diagrams of the control
strategy;
[0042] FIGS. 15(a)-(f) are graphs of the performance of different
control modes;
[0043] FIGS. 16(a) and 16(b) are graphs of the simulated results of
an embodiment;
[0044] FIG. 17 is block diagram of an prototype according to an
embodiment;
[0045] FIG. 18 is a photo of the prototype in FIG. 17;
[0046] FIG. 19 is a table of the parameters for the prototype in
FIG. 17;
[0047] FIGS. 20-24 are graphs of the performance of the prototype
in FIG. 17,
[0048] FIG. 25 is a table comparing the connection arrangements;
and
[0049] FIG. 26 is a flow diagram of a control algorithm according
to an example embodiment.
DETAILED DESCRIPTION
[0050] The following documents are incorporated herein by
reference:
[0051] Singapore patent application number 201004447-7 filed 21
Jun. 2010; Zhou, Bhattacharya, Khambadkone, "Composite energy
storage system using dynamic energy management in micro-grid
applications", The 2010 International Power Electronics Conference,
ECCE Asia, 2010;
[0052] Tran, Zhou, Khambadkone, "Energy management and dynamic
control in composite energy storage system for micro-grid
applications" IECON-2010, 11 2100; and
[0053] Haihua Zhou; Bhattacharya, T.; Duong Tran; Siew, T. S. T.;
Khambadkone, A. M.; "Composite Energy Storage System Involving
Battery and Ultracapacitor With Dynamic Energy Management in
Microgrid Applications", Power Electronics, IEEE Transactions on
Volume: 26, Issue: 3 10, 2011, Page(s): 923-930.
[0054] An energy system 400 according to the embodiment shown in
FIG. 4(a) is now described. The system includes a load 402, a DC
bus 404, a PV module 406, a composite energy storage system (CESS)
408 and a controller 410. The load 402 may be a single phase
residential load, or in larger scale implantations may be a 3 phase
high voltage diversified load. The PV module 406 may be building
integrated, such as roof mounted, and will provide a DC voltage and
current that varies considerable over the daytime, as sunlight
conditions determine. The CESS 408 may store energy or supply
energy, depending on the load 402 relative to the supply from the
PV module 406. Thus typically the current flows from the PV module
406 through the DC bus 404 to the load 402. The CESS 408
instantaneously resolves any mismatch.
[0055] The physical installation of the energy system 400 is shown
in FIGS. 4(b) and 4(c). The PV module 406 may be installed on the
roof 412 of the building 414. A protection system and cabling 416
connect the PV module 406 to the DC bus 404 in a cool dark
location. The controller 410 and CESS 408 are connected adjacent to
the DC bus 404. The load 402 is distributed throughout the building
and is consolidated at a distribution board connected to the DC bus
404.
[0056] As shown in FIG. 5 the CESS 408 according to the embodiment
in FIG. 4, is easily reconfigurable to a desired operating region
500. This allows the CESS 408 flexibility to supply a far greater
range of generation/load scenarios.
[0057] The particular choice of configuration for the CESS 408 may
depend on the generation/load scenario and the application or
design parameters. Such parameters may include the maximum power,
the maximum energy, the maximum weight, the maximum volume, the
maximum ripple or EMI level, load type (single or three phase),
output voltage level, desired life expectancy, desired level of
reliability, level of control complexity, grid control integration
and cost. The configuration will relate to a particular combination
of different storage mediums together with a particular control
strategy that may be implemented in hardware, software or a
combination of the two.
[0058] For example as shown in FIG. 6 a load profile 600 is shown
with a peak power requirement 602 of 3 kW and an energy requirement
of 6 kWh over each 24 hour period. Thus as shown in FIG. 7, if the
CESS 408 was implemented using just a fuel cell 700, the weigh
would be 150 kg. If the CESS 408 was implemented using just an
ultracapacitor 702, the weigh would be 600 kg. For the load profile
600 if the optimum configuration 704 was found to be a 6 kWh fuel
cell combined with a 3 kW ultracapacitor, the weight would only be
31.2 kg. The choice of which mix of storage medium to use may
depend on how volatile or dynamic the generation/load profiles are,
cost and the overall power and energy requirements.
[0059] The CESS 408 is shown in more detail in FIG. 8. The PV
module 406 includes an array of solar cells 800, connected to the
DC bus 404 via a DC-DC converter 802. Energy generation may come
from other sources such as a wind turbine, micro hydro, combined
cycle turbine/generator or an electric vehicle 804 configured for
emergency power generation mode. In larger scale installations,
different energy generation and/or storage mediums might be
appropriate.
[0060] The load 402 may be connected to a first DC voltage port
806, a second DC voltage port 808 or a third AC voltage port 810.
The first port 806 may be directly connected to the DC bus 404, and
thus at a higher voltage than the second port 808. The DC-DC
converter 802 and solar cell 800 may be connected to the second
port 808. The electric vehicle 804 may be connected to either the
first port 806 or the third port 810. The third port 810 is
connected to the DC bus 404 via a DC-AC converter 811. The electric
vehicle 804 may also be a load, connected to the first port 806 or
the third port 810 for charging.
[0061] As mentioned previously the choice of storage mediums
depends on a number of parameters. In FIG. 8 the CESS 408 is shown
with a first battery 812, a second battery 814, an ultracapacitor
816 and a flywheel 818. The first battery 812, second battery 814,
and ultracapacitor 816 are connected to the DC bus 404 via
respective DC-DC converters 819,820,822. The flywheel 818 is
connected to a AC generator / motor 824 (such as a brushless DC
motor) which connects to the DC bus 404 via an DC-AC converter
826.
[0062] The DC-DC converters 802,819,820,822 are bidirectional
isolated DC-DC converters such as a dual active bridge (DAB)
converter 900 as shown in FIG. 9. The DAB 900 is bidirectional,
isolated, soft switching with a high frequency transformer 902.
This allows for a much smaller transformer 902 and filter.
Isolation allows the DABs 900 to be connected up in series,
parallel or a combination according to different load requirements.
Such different connection arrangements are shown in FIGS. 10(a) to
10(h) including input parallel output parallel (IPOP), input
parallel output series (IPOS), input series output series (ISOS)
and modifications of these. The chosen configurations may also be
affected by the relative price of different voltage batteries,
ultracapacitors etc. FIG. 25 shows a comparison of the connections
arrangements and possible applications for each. Similarly the
switching stress is shown in FIG. 12 for each configuration, which
will affect the capacity and therefore cost of the switches
used.
[0063] The DAB 900 includes a first full bridge AC inverter 904,
connected in series with a filter inductor 906 and the primary
winding of the high frequency transformer 902. The transformer 902
provides isolation, but due to high frequency switching its size
can be minimised. The secondary winding of the transformer 902 is
connected to a full bridge AC-DC converter 908, with a filter
capacitor 910 connected at the output.
[0064] The modified IPOS connection configuration from FIG. 10(d)
is shown in more detail in FIG. 11. Each battery 812,814,1100,1102
is independently connected to a respective DAB 819,820,1104,1106,
input. The DAB 819,820,1104,1106 outputs are connected in series to
give a DC bus 404 voltage of 800V. This 800V configuration might be
used where a 3 phase 440V AC supply is required. Similarly the
ultracapacitor 816 is connected in parallel to a series of DABs
822,1112,1114,1116, with their outputs also connected in series to
the DC bus 404 and are intermediately bridged to the outputs of
each respective battery DABs 819, 820, 1104, 1106. The switching
instants of each DAB 819, 820, 1104,1106, 822, 1112, 1114, 1116 may
be interleaved to reduce the input current ripple 1300 from 222% to
47 as shown in FIG. 13(a) and the voltage output ripple 1302 from
0.67% to 0.08% % measured at the second port 808 as shown in FIG.
13(b).
[0065] DC-AC converters 811,826 may be 3 phase (single phase if low
power) inverters.
[0066] The DABs can be reconnected in a different connection using
external terminals that could be connected as desired using
removable jumper cables or using insulated bus-bars.
[0067] The controller 410 may be implemented according to FIGS.
14(a) to 14(c). As mentioned earlier the control strategy may be
hardware based, software based or a combination of the two. For the
example where the energy storage is batteries and an
ultracapacitor, a hardware control implementation is shown in FIG.
14(a). The controller 410 receives the State of Charge
(SOC.sub.1-n) of each storage medium, the PV module 406 power
P.sub.PV and the load 402 power P.sub.L. Subtracting the P.sub.L
from the P.sub.PV gives the current demand P.sub.CESS for the CESS
408. The P.sub.CESS is divided into a low frequency component
P.sub.BAT to control the batteries 812,814 and a high frequency
component P.sub.UC to control the ultracapacitor 816, by a low pass
filter (LPF).
[0068] In FIG. 14(b) the control logic is shown to control the
modified IPOS connection configuration in FIG. 11. Effectively each
DAB has an individual current demand, and there is a current
feedback loop for each DAB 900 to control the current. Each DAB 900
implements the current control by adjusting the relative phase
between the inverter 904 and the converter 908.
[0069] The current demand for the ultracapacitor DABs is
fed-forward to the output of a voltage feedback loop. The
ultracapacitor DABs are more directly responsible for regulating
the voltage as they have the responsiveness to do so, while the
battery DABs are primarily responsible for maintaining the steady
state power balance.
[0070] The current demands for each DAB are determined by the
controller 410 according to which operational mode is been
selected. For example the modes might include a) dynamic
distribution of the current demand between the batteries and the
ultracapacitor, b) distributing the SOC between the batteries, c)
replacement of a battery, and d) charging of the
ultracapacitor.
[0071] For mode (a), as can be seen in FIG. 14(c) the P.sub.BATT
(or in this case I*.sub.i) current demand for the batteries is
simply split equally to each battery DAB 819, 820, 1104,1106.
Similarly the P.sub.UC (or in this case I*.sub.h) current demand
for the ultracapacitor is simply split equally between each DAB
822, 1112,1114,1116. The voltage feedback and current feedback
formulas are shown in Equation 1 and 2:
C.sub.v=K.sub.cv(1+1/sT.sub.iv) (1)
C.sub.i=K.sub.ci(1+1/sT.sub.ii) (2)
[0072] The cut off frequency of the LPF is determined based on the
relative capacities of the batteries and the ultracapacitor. The
formula for the LPF is shown in Equation 3:
G f = 2 .pi. f cf s + 2 .pi. f cf ( 3 ) ##EQU00001##
[0073] The calculation of the overall current demand and the
separation into high and low frequency components (shown in FIG.
14(c)) may be carried out inside the controller 410 in FIG.
14(b).
[0074] FIG. 15(a) shows a simulation of the performance of the
controller in FIG. 14(c) with a step change in the P.sub.CESS
current demand. Initially the increased current is supplied by the
ultracapacitor, and after about 20 ms the increased current is
supplied in the steady state by the batteries. This frequency
response based decomposition of current reference can be tuned to
the frequency response of the storage medium either manually or
automatically.
[0075] For mode (b), as seen in FIG. 15(b) if the current demand is
evenly shared to all batteries and one battery starts with a lower
SOC than the rest, the batteries with higher SOC cannot be fully
utilised. Flexible distribution of SOC is shown in FIG. 15(c),
where the individual current demands to the batteries are varied to
achieve SOC balance.
[0076] For mode (c), as seen in FIG. 15(d) when battery 812 is
removed, the DAB 819 current tends to zero, DABs 820, 1104, 1106
share the shortfall, and ultracapacitor DAB 822 temporarily charges
DABs 1112, 1114,1116,. As shown in FIG. 15(e) the impact on the DC
bus 404 voltage is minimal.
[0077] For mode (d), as seen in FIG. 15(f) the ultracapacitor
voltage is monitored and if it falls below a threshold, a negative
current demand is sent to DABs 1112, 1114,1116, to charge the
ultracapacitor and a positive current demand is sent to the DABs
819, 820, 1104,1106 to maintain the DC bus 404 voltage.
[0078] As shown in FIGS. 16(a) and 16(b), the output from the PV
module 406 is volatile. As a result the P.sub.CESS includes a
significant high frequency component. The ultracapacitor is able to
provide the fast response to changes in demand, and the steady
state shortfall is provided by the batteries.
[0079] The controller may be programmed with the algorithm 2600
shown in FIG. 26. The algorithm 2600 receives the inputs SOC (for
each storage medium) P.sub.PV and P.sub.Load and determines the
current demands for each DAB. As shown in FIG. 14(b) the current
demands are integrated into the current and voltage feedback
loops.
[0080] A prototype according to an embodiment is shown in FIGS. 17
and 18. In this setup there are two batteries 1700,1702 and an
ultracapacitor 1704 with a 200V DC bus 1706 voltage. The controller
is implemented in a digital signal processer and field programmable
gate array (FPGA) 1900 shown in FIG. 19, using the parameters shown
in FIG. 20. FIG. 21 shows the relatively good performance in the DC
bus 1706 voltage with a step load change from 300 W to 600 W. FIG.
22 shows the relatively good current performance and FIG. 23 shows
the relatively good voltage performance when a battery is removed.
Lastly FIG. 24 shows the relatively good performance at charging
when the ultracapacitor voltage falls below the threshold.
[0081] One or more embodiments may be applied in a micro grid
configuration, stand alone or island configuration or as the energy
storage in an electric vehicle. Embodiments may have the advantages
of independent dynamic allocation of steady state and transient
power demand to different storage mediums, flexible distribution of
power flow between different batteries, online battery replacement,
continuous ultracapacitor charging or discharging, independent
upgrading or power and energy capacity and/or modularity.
* * * * *